1. Field of the Invention
The present invention relates to an image pickup apparatus which photoelectrically converts a light beam from an imaging optical system.
2. Related Background Art
In a digital camera, a subject image is exposed to a solid-state image pickup element such as a CCD or CMOS sensor for a desired time in response to depression of a release button. The resultant image signal that represents a still image of one frame is converted into a digital signal and subjected to predetermined processing such as YC processing, thereby obtaining an image signal having a predetermined format. A digital image signal representing a pickup image is stored in a semiconductor memory on each image basis. A stored image signal is read out and reproduced into a displayable or printable signal and output and displayed on a monitor or the like as needed.
Conventionally, focus detection in the image pickup optical system of a digital camera employs a contrast detection scheme using an image pickup apparatus. In such focus detection of a contrast detection scheme, generally, since the extremal value of contrast is obtained while slightly moving the on-axis position of the image pickup optical system, a considerably long time is required for focus adjustment until an in-focus state is obtained.
A focus detection method in which focus detection of a phase difference detection scheme used for, e.g., a single-lens reflex camera using a silver halide film is performed using the image pickup element of a digital camera has been proposed. In focus detection of the phase difference detection scheme, since the defocus amount can be obtained, the time required to obtain an in-focus state can be greatly shortened as compared to the contrast detection scheme. An example of such focus detection method has been proposed, in which a pair of photoelectric conversion units are prepared for each of microlenses that are two-dimensionally arrayed, a pixel unit formed from the pair of photoelectric conversion units is projected to the pupil of an image pickup optical system to separate the pupil, thereby executing focus detection of the phase difference scheme. In addition, an image pickup apparatus using, as a light-receiving means, a solid-state image pickup element described in Japanese Laid-Open Patent Application No. 9-46596 which can arbitrarily switch between addition and non-addition of a pair of photoelectric conversion unit outputs in one microlens has been proposed.
FIG. 39 is a sectional view showing a pixel unit in this image pickup apparatus.
Referring to FIG. 39, the pixel unit comprises a p-type well 124, gate oxide films 125 and 126, polysilicon layers 127 and 128, n-type layers 129 and 130 with a concentration capable of complete depletion, an n+-type floating diffusion region (FD region) 131, and p+-type surface layers 132 and 133. The FD region 131 is connected to the n-type layers 129 and 130 serving as first and second photoelectric conversion units, through the polysilicon layers 127 and 128 as the components of a transfer MOS transistor. The n-type layers 129 and 130 and p+-type surface layers 132 and 133 form photoelectric conversion units as buried photodiodes. With this structure, a dark current generated on the surface can be suppressed. A color filter CF passes light in a specific wavelength range. A microlens μL efficiently guides a light beam from the image pickup optical system to the first and second photoelectric conversion units. The power of the microlens μL is set such that the exit pupil of the image pickup optical system and the pair of photoelectric conversion units in each pixel unit form images. Hence, the first and second photoelectric conversion units are designed to photoelectrically convert light components that have passed through different regions on the exit pupil.
FIG. 40 shows the state of exit pupil separation in the image pickup optical system by the first and second photoelectric conversion units. Referring to FIG. 40, a virtual image 1 is obtained when an iris ST is in a full-aperture state, and the aperture of the iris ST is viewed through rear lens groups grp3 and grp4. A hatched portion 2 indicates the first region on the exit pupil through which a light component that becomes incident on the first photoelectric conversion unit of a solid-state image pickup element 100 passes. A hatched portion 3 indicates the second region on the exit pupil through which a light component that becomes incident on the second photoelectric conversion unit of the solid-state image pickup element 100 passes. A small gap 4 is present between the first region 2 and the second region 3. A light component that has passed through this region is photoelectrically converted by neither of the first and second photoelectric conversion units. This is because the n+-type FD region is inserted between the buried photodiodes formed from the n-type layers 129 and 130 and p+-type surface layers 132 and 133, as shown in FIG. 38. At this portion, photoelectric conversion is not performed.
With the above arrangement, the solid-state image pickup element can independently transfer charges generated in the first and second photoelectric conversion units to the FD region 131. Only by adjusting the timings of transfer MOS transistors connected to the FD region 131, switching between addition and non-addition of signal charges of the two photoelectric conversion units is realized. Hence, in the image pickup mode, the signal charges of the first and second photoelectric conversion units are added and read, thereby photoelectrically converting the light components from the entire exit pupil of the image pickup optical system.
At the time of focus detection, the signal charges of the first and second photoelectric conversion units are independently read, thereby independently photoelectrically converting light components that have passed through different regions on the exit pupil of the image pickup optical system. Since the exit pupil of the image pickup optical system is separated into the first region 2 and second region 3, as shown in FIG. 40, the direction in which object images generates a phase shift due to defocus of the image pickup optical system is indicated by an arrow A. Hence, at the time of focus detection, a pixel array whose longitudinal direction is set in the direction indicated by the arrow A is set on the solid-state image pickup element, and a pair of image signals are generated from a pair of signal charges that are independently read, thereby detecting the phase shift between object images. To detect a phase shift, known correlation calculation is used. With the above arrangement, both image pickup and focus detection using the phase difference scheme are realized by the solid-state image pickup element 100.
In focus detection using the phase difference scheme, to accurately detect a phase shift from small to large defocus, the pair of object images in the direction of phase shift at an arbitrary defocus amount preferably have almost similar shapes. In this case, the phase shift amount between the pair of object images, which is calculated by known correlation calculation, and the defocus amount of the image pickup optical system can have an almost linear relationship. Hence, the defocus amount of the image pickup optical system can easily be derived from the phase shift amount.
Letting f(x,y) be the light amount distribution of a subject image and g(x,y) be the light amount distribution of an object image, a relationship (convolution) given by
                              g          ⁡                      (                          x              ,              y                        )                          =                              ∫                          -              ∞                        ∞                    ⁢                                    ∫                              -                ∞                            ∞                        ⁢                                          f                ⁡                                  (                                                            x                      -                      a                                        ,                                          y                      -                      b                                                        )                                            ⁢                              h                ⁡                                  (                                      a                    ,                    b                                    )                                            ⁢                                                          ⁢                              ⅆ                a                            ⁢                                                          ⁢                              ⅆ                b                                                                        (        1        )            holds. In this case, h(x,y) is a transfer function representing a state wherein a subject image degrades in an image forming system, which is called a point spread function. Hence, to know the similarity between a pair of object images to be used for focus detection, the point spread function must be known.
In focus detection using the phase scheme, since the phase shift between a pair of object images is detected with an emphasis on their one-dimensional direction, the image system related to focus detection can be evaluated using not a point spread function but a line spread function as a one-dimensional function. Then, replacing light amount distribution of the subject image with f(x), replacing the light amount distribution of the object image with g(x), and using a line spread function L(a), equation (1) can be rewritten to
                              g          ⁡                      (            x            )                          =                              ∫                          -              ∞                        ∞                    ⁢                                    f              ⁡                              (                                  x                  -                  a                                )                                      ⁢                          L              ⁡                              (                a                )                                      ⁢                                                  ⁢                          ⅆ              a                                                          (        2        )            
When a pair of line spread functions in the phase shift direction at the time of defocus are known from equation (2), the similarity between the pair of object images can be known, and the basic defocus performance of focus detection using the phase difference scheme can be known. As is apparent from equation (2), the higher the similarity between the pair of line spread functions becomes, the higher the similarity between the pair of object images becomes.
For the intensity distribution of point images formed on the imaging surface by a given point light source that has passed through the exit pupil of the optical system, i.e., so-called point spread function, it can be regarded that the exit pupil shape is reduced and projected onto the imaging surface. Similarly, for the line spread function, it can be regarded that the exit pupil shape in the one-dimensional direction, i.e., a shape obtained by integrating the exit pupil shape in the one-dimensional direction is reduced onto the imaging surface through the microlens μL. In fact, due to the aberrations or manufacturing error in optical system, the imaging position or intensity changes depending on the passage position of a light component on the exit pupil and the shape of the line spread function slightly changes. However, since the purpose here is to know the similarity between the pair of line spread functions in the phase shift direction, the image pickup optical system and microlens μL will be simplified as ideal lenses without any aberrations. In addition, the sensitivity distributions of an infrared cut filter F1, low-pass filter LPF, color filter CF, and photoelectric conversion units and the S/N ratios of the photoelectric conversion units, which have been described in the prior art, will also be omitted.
The first and second regions 2 and 3 through which light components incident on the first and second photoelectric conversion units in FIG. 40 pass are integrated in the phase shift direction, i.e., the direction indicated by the arrow A. FIG. 41 is a graph showing the integration result in which an optical axis L1 is set at the origin. The abscissa represents the phase shift direction, and the ordinate represents the intensity. A first pupil intensity distribution 5 corresponds to the first region 2 on the exit pupil, and a second pupil intensity distribution 6 corresponds to the second region 3 on the exit pupil. Actually, the first and second pupil intensity distributions are reduced onto the imaging surface through the microlens μL to form line spread functions, but the similarity between the pair of line spread functions can be known from this graph. The defocus of the image pickup optical system is not taken into consideration here. For a small defocus, it can be regarded that the first and second pupil intensity distributions 5 and 6 are reduced in the abscissa direction and enlarged in the ordinate direction. For a large defocus, it can be regarded that the pupil intensity distributions are enlarged in the abscissa direction and reduced in the ordinate direction.
Referring to FIG. 41, the first and second pupil intensity distributions 5 and 6 have semi-circular shapes mirror-inverted in the phase shift direction, i.e., abscissa direction, and the similarity therebetween is low. Hence, it is known that the pair of line spread functions at an arbitrary defocus amount of a light beam passing through the first region 2 on the exit pupil of the image pickup optical system also have a low similarity in the phase shift direction. The line spread functions near the in-focus state are obtained by extremely reducing the pupil intensity distributions 5 and 6 in the abscissa direction and enlarging them in the ordinate direction. That is, the line spread functions have shapes like impulse waveforms, and therefore the similarity becomes high. However, when the image pickup optical system defocuses to some degree, the mirror-inverted semi-circular shapes conspicuously appear, and the similarity decreases.
When the image pickup optical system defocuses to some extent, and the similarity between the pair of line spread functions becomes low, as indicated by the first and second pupil intensity distributions 5 and 6 in FIG. 41, the pair of object images on the imaging surface nonuniformly deform in the phase shift direction because of the influence of the line spread functions, and the similarity becomes low. On the other hand, when the image pickup optical system is almost in the in-focus state, the line spread functions exhibit shapes like impulse waveforms. For this reason, the pair of object images on the imaging surface also exhibit almost similar shapes. The object images in the phase shift direction are equivalent to image signals obtained by pixel arrays whose longitudinal direction is set in the phase shift direction. Hence, when the image pickup optical system defocuses to some degree, the pair of image signals obtained by the first and second photoelectric conversion units have a low similarity and exhibit shapes inverted in the horizontal direction.
In focus detection using the phase difference scheme, the phase shift amount calculated by known correlation calculation, and the defocus amount of the image pickup optical system are made to have an almost linear relationship therebetween in a practical defocus range, and an expected defocus amount is calculated from the detected phase shift amount, thereby setting the image pickup optical system in the in-focus state. In the prior art, the phase shift amount and defocus amount have an almost linear relationship therebetween in a small defocus range wherein the pair of image signals have a high similarity. However, as the defocus amount increases, the similarity between the pair of image signals decreases, so the relationship between the phase shift amount and the defocus amount cannot be linear. When the image pickup optical system defocuses to some extent, the in-focus state cannot be obtained by executing focus detection only once. However, since an almost linear relationship is obtained in a small defocus range, the in-focus state can be obtained through the small defocus state by executing focus detection a plurality of number of times.
To do this, the iris ST of the image pickup optical system is designed as a two-aperture iris which has a pair of aperture portions in focus detection and retreats at the time of image pickup. In this case, since the characteristics of the pair of line spread functions in focus detection can be improved, focus detection need not be executed a plurality of number of times even in a large defocus state.
In the above prior art, however, especially when the image pickup optical system defocuses to some extent, focus detection must be executed a plurality of number of times. This makes it impossible to quickly adjust the focus state as an advantage of focus detection using the phase difference scheme.
When the above-described two-aperture iris having a pair of aperture portions is used, focus detection need not be executed a plurality of number of times even when the image pickup optical system defocuses to some extent. However, since the image pickup optical system must have an iris drive mechanism that causes the iris to retreat, the image pickup apparatus becomes bulky and expensive.
When the amount of light incident on the pair of photoelectric conversion units and the aperture ratio of the iris ST of the image pickup optical system have an almost linear relationship therebetween, and the luminance of the object and the sensitivity of the image pickup element are given, so-called APEX calculation to calculate the F-number and shutter speed according to the same procedure as in a film camera can be executed. In this case, since the exposure amount can be calculated using a general exposure meter, like a film camera, the phototaking operation is very easy. In the prior art, however, since the n+-type FD region 131 is formed between the pn photodiodes 129 and 130, as shown in FIG. 39, the gap 4 where a light component is photoelectrically converted by neither of the first and second photoelectric conversion units is present. When the iris ST is set in a stopped-down-aperture state, the ratio of the gap 4 to the aperture region of the exit pupil increases. For this reason, the amount of light incident on the pair of photoelectric conversion units and the aperture ratio of the iris ST of the image pickup optical system have no linear relationship therebetween. As the F-number increases, the error increases. Hence, the exposure calculation using APEX also has an error, and no general exposure meter can be used.
Even from the viewpoint of image pickup, if a region where photoelectric conversion is not performed is present between the first and second photoelectric conversion units, an unnatural blur (double-line effect) readily occurs in an obtained image.